surface has this same radius, the refracting power is 

 one-half and the focus is twice as long, or two inches. 



In a concave lens, the action of the lens is opposite; 

 instead of converging the light toward the axis, it 

 diverges the rays from the axis as shown in Fig. 8. 



Fig. 8. 



The imaginary extension of the diverging rays 

 should meet at e and the distance t b indicates the 

 virtual focus, in contradistinction to the real focus in 

 the convex lens. 



To Determine the Focal Length of a 

 Convex Lens. The focal length of the lens may 

 be quite accurately determined by the following 

 methods : 



